A new category of tropospheric aeolian generators is known in the art, currently being developed by different search groups, that share the common objective of exploiting the great amount of aeolian energy at high altitudes through kites, wings, aircrafts, aerostats and airships constrained to the ground through long ropes with high mechanical resistance.
The common operating principle of the tropospheric aeolian generators is based on keeping flying aerodynamic bodies that are able to convert wind energy at high altitudes into mechanical energy capable of performing works and, afterwards, on converting mechanical energy into electric energy that can be used for civil and industrial purposes in general.
In its simplest, most efficient and safe configuration, the aerodynamic body can simply be a wing with high aerodynamic efficiency, kept flying at heights that cannot be reached by current aerogenerators and constrained to the ground through high-resistance ropes.
Alternatively, the aerodynamic body can be much more complex, for example an aeolian turbine rotor kept flying due to an airship, or an aircraft equipped with tail planes and stabilising members.
In particular, all generators in this category are equipped with at least one constraining rope that is periodically wound and unwound through a winch or a system of winches.
Not only generators that exploit the winch rotation to convert mechanical energy into electric energy, but also generators in which the rope winding and unwinding is used only for checking the flight height and the trajectory or, still more simply, only for takeoff and landing phases, can find advantages in the present invention.
Also naval traction systems or electric generation systems aboard of ships and vessels based on kites can find advantages in the present invention.
Examples of such aeolian generators are disclosed in Italian patents n. 0001344401 and 0001344926 in the name of Ippolito Massimo, that describe the general concept on which a tropospheric aeolian generator is based, in European patent n. EP1672214 in the name of Ippolito Massimo, that describes the carousel-type configuration, in PCT Patent Application n. PCT WO2007/129341 in the name of Kite Gen Research S.r.l., that describes the control system, and in Italian Patent Application n. TO2008A000423, that describes the generator infrastructure in its configuration called “yo-yo”.
In particular, energy that can be extracted through known generators mentioned above depends firstly on atmospheric phenomena, such as wind speed and direction, that can be evaluated upon designing, but cannot be affected by the designer. Energy that can be extracted however also depends on accurate design choices that can be controlled, such as surface and aerodynamic characteristics of the sails.
All evaluations performed by the Applicant on energy that can be extracted through known generators mentioned above have confirmed that, among the parameters that remain at the designer's discretion, the aerodynamic efficiency of the global system composed of wing and ropes is the most important element: in fact, the aerodynamic efficiency appears raised to a power of two in formulas that describe and foresee the energy that can be collected, while the sail surface appears linearly. The sail efficiency, represented by the ratio between lift coefficient and resistance coefficient, is generally high, due to the sail' aerodynamic section that brings about a low value of the aerodynamic resistance coefficient.
In order to improve safety and reliability of generators disclosed by the Applicant, the adoption of a pair of ropes has been chosen, instead of using a single rope as preferred by others, such as disclosed, for example, in U.S. Patent Application 2008/0210826 of Ockels et al. In fact, the pair of ropes allows first of all to control the wind trajectory without the need of installing electro-mechanical components on board, and protects the manoeuvre capacity from malfunctions, failures, communication difficulties of possible components installed on board the kite, in addition to the uncontrolled fall and loss of the wing in case of breakage of the single rope.
The pair of ropes further transform the rare event represented by the breakage of a rope, for example due to a manufacturing defect, from a potentially dangerous event into a simple recovery and maintenance procedure: the breakage of one of the two ropes in fact implies the instantaneous decrease of wing lift, with following reduction of the stress acting on the remaining rope. In this way, it is always possible to bring back the wing to the ground, by quickly rewinding the remaining rope, due to the behaviour of the wing that can be assimilated to a parachute.
The same principle is also adopted and advantageously exploited under operating conditions during the rope re-winding step that follows the winding and energy generating step: in fact, by releasing in a controlled way one of the ropes and by keeping tension on the second rope, the wing is naturally taken to a position for which the resultant of aerodynamic forces is composed almost exclusively by the resistance, while the lift becomes neglectable. By keeping the wing in this particular attitude, that could be defined as “sideslip” manoeuvre, as analogy with what is performed under emergency situations or during a fight by aircraft pilots, it is possible to high-speed re-wind the control ropes with a minimum energy cost.
The use of two ropes therefore implies an increase of the global resistance with respect to the solution with a single rope, but provides undoubted advantages in terms of safety and reliability.
The commercially available ropes are however not conceived, and consequently optimised, to be used by a tropospheric aeolian generator and are a severely limiting factor of global aerodynamic performances.
As known, the behaviour of ropes in simulations can be ascribed, as first approximation, to the behaviour of a smooth cylinder with infinite length crossed by a current orthogonal to the cylinder axis, estimating a resistance coefficient typically equal to CD=1,2 depending on experimental data in the wind gallery related to the number of Reynolds typical of many practical applications. This approach, that does not take into account real cable shape, surface roughness, longitudinal and torsion elasticity, implies the under-estimation, in general, of the real entity of the resistance coefficient.
For example, a rope with many strands can have a still greater resistance coefficient, equal to CD=1,5 when immersed in a uniform fluid. In case of long cables, when the phenomenon of vibration induced by vortexes (VIV) is triggered, the resistance coefficient can even reach values on the order of CD=2.5-3, as well as forces can appear that are orthogonal to the current to which a lift coefficient CL corresponds. The problem is particularly sensible in many application fields, in particular in the naval and offshore fields, where particularly long ropes and cables are subjected to the action of currents with varying intensity, for example the tie-rods of oil platforms, or cables that tow remote-controlled submarine vehicles (ROV).
There are obviously also many examples in the civil field, where suspended lines for transmitting electric energy, or tie-rods of suspended bridges, can be affected by potentially dangerous oscillations. In these applications, solutions are first of all searched that are able to reduce the oscillation amplitude and make it unlikely that instabilities of the elastic balance occur, and efficiency is required independently from the direction of the incident fluid.
In other applications above all a reduction of the cable fluid-dynamic resistance is searched.
Among the many proposed methods, it is possible to remember the roughness distributions according to repeated schemes, the distribution of surface bumps or recesses, the helical windings, the addition of bands of fabric, more or less aerodynamic rigid or flexible fairings.
However, when the cable is subjected to repeated winding and unwinding cycles on a winch drum, as occurs in aeolian generators of the previously described types, the complexity of the technical problem increases and the choice of available solutions is reduced.
Also in aeronautics, the problem is particularly relevant, since already at the time of biplanes, in England profiled structural members were developed, called “RAF wires”, as replacement of tie-rods made of steel cables or full red iron. The importance of such phenomenon can be evaluated starting from considerations about dimensioning a typical tropospheric generator. An aircraft wing can generate a lift on the order of 10 kN/m2.
For example, a fully loaded Boeing 747-400 upon takeoff has a ratio between weight and wing surface equal to W/S=7500 N/m2. Since this is an aircraft for transporting passengers, in which accelerations must be limited (load factor n=2.5), it can be deduced that the wing structure is sized to tolerate, under safety conditions, a specific lift equal to 18750 N/m2.
A reference value for the ratio between weight and wing surface of an aircraft without engine, such as for example gliders, hang gliders, crazy fly crafts and kites can be on the order of W/S=300 N/m2.
These aircrafts, in spite of the structure of their wings, weighs about 50 N/m2, however they can perform manoeuvres with a high number of g (load factor n=6 for the acrobatic category), and therefore can develop and tolerate specific lift values on the order of 1800 N/m2.
Assuming for the kite of the generator devised by the Applicant and described in the above patents, a lift coefficient CL=1, a density value ρ=1.225 kg/m3, a wing surface S=100 m2, a flight speed V=40 m/s, the classical lift formula returns the value of 98000 N, therefore a value near 1000 N/m2.
Assuming for example F=100 kN as value of the force generated by the kite that has to be transmitted to the generator through the ropes, it is clear that these latter ones must be suitably sized.
Examining the tables of the best manufacturers of synthetic ropes, assuming to use a single rope (solution A) optimised for repeated flexure cycles and adopting a safety coefficient S=3, a single rope would be necessary whose diameter is D(a)=18 mm with ultimate tensile stress of 304 kN.
Assuming instead to use a pair of ropes (solution B), due to the reliability and safety reasons that have been stated before, two ropes would be necessary, having a diameter D(b)=14 mm and ultimate tensile stress of 168.6 kN for a whole 337 kN. In fact, assuming very approximately, but with a typical behaviour, that the rope moves with null speed with respect to air next to the ground generator and with a speed equal to the kite speed next to the kite itself, with a linear speed variation along the rope, it is possible to estimate the global rope resistance.
By adopting an aerodynamic resistance coefficient CD=1.2 and a flight speed equal to V=40 m/s, a single rope with diameter D(a)=18 mm and length 1000 m completely unwound generates a global resistance next to 7054 N. Under the same conditions, a rope with diameter D(b)=14 mm generates a global resistance next to 5487 N, therefore taking into account a pair of ropes having diameter D(b)=14 mm, the global resistance is next to 10974 N.
It is clear that, from the point of view of the aerodynamic resistance, both above solutions A and B would anyway be problematic, with a disadvantage for the solution with two ropes (solution B) that is anyway amply justified by the increase of safety and reliability.
In view of the above, the art has proposed several solutions suitable to improve the cyclic fatigue resistance of ropes subjected to repeated flexure around pulleys and winches.
For example, PCT Patent Application WO2004/035896 to Knudsen R. B and Sloan F. E. discloses a rope built using a mixture of filaments of a different nature, in this case HMPE and LCP, according to a particular proportion.
PCT Patent Application WO2005/019525 to Frazer et al. discloses a rope with a core without structural function that fills the empty space between strands required to support the loads.
Similarly, PCT Patent Application PCT WO2006/086338 to Bucher et al. innovates by introducing fibres with low friction coefficient, in particular fluoro-polymeric fibres, to compose the rope strands.
A similar solution is described in PCT Patent Application WO2006/101723 to Nye, in which a filament of fluoro-carbon polymer is used.
PCT Patent Application WO2006/133881 to Bosman R. instead discloses a rope in which the transverse section is oblong, having a fineness ratio included between 1.2 and 4.0, as well as a pulley with groove adapted to the rope section.
In all cases, the purpose is increasing the number of useful life cycles of the rope without excessively increasing the rope diameter and weight, at the same time keeping the chance of performing a visual inspection that points out the wear status and possible localised damages.
Several solutions have also been proposed to reduce the fluid-dynamic wear of ropes that relatively move with respect to a fluid, almost all for applications in the naval and oceanographic fields, therefore with needs and adopted solutions that are very different from those suitable for a tropospheric aeolian generator.
For example the following can be cited: patent CA887428 to Pearce et al, U.S. Pat. No. 3,859,949 to Toussaint and Meyer, U.S. Pat. No. 4,365,574 to Norminton, U.S. Pat. No. 4,836,122 to Henderson and Wingham, U.S. Pat. No. 6,179,524 to Allen et al, PCT Patent Application WO2005/116459 to Allen et al., PCT Patent Application WO2006/134381 to Pearce, U.S. Pat. No. 6,179,524 to McMillan, U.S. Pat. No. 6,223,672 to Allen et al. as example of covers, fairings, profiles suitable to suppress the vibrations induced by vortexes and reduce the aerodynamic resistance of cylindrical bodies immersed in a sea environment.
Interesting examples are then U.S. Pat. No. 4,084,065 to Swenson and the more recent U.S. Pat. No. 5,067,384 to Scala that describe how to make a cable equipped with a braid in which a series of filaments are free to be oriented in the current and reduce induced vibrations and aerodynamic resistance.